The invention relates to a method for preparing diene polymers or copolymer having increased amounts of 1,2-microstructure in the diene units thereof. More particularly, the invention relates to the polymerization of diene monomers or copolymerization of diene monomers and vinyl aromatic hydrocarbon monomers in the presence of a catalyst composition which comprises an organolithium initiator and an oxolanyl cyclic acetal modifier to produce such polymers and copolymers.
Methods of preparing diene polymers or copolymers having increased amounts of 1,2-microstructure in the diene units using an organolithium initiator and various modifier compounds are known in the art as illustrated by the following patents.
U.S. Pat. No. 3,207,742 describes a process for preparing rubbery polymers of 1,3-dienes having an increased ratio of 1,2-microstructure. Such polymers are obtained by conducting the polymerization of the 1,3-diene in the presence of a lithium hydrocarbon initiator and a hexamethylphosphoramide modifier. The amount of the phosphoramide present is from about 10 to 0.1 mole percent based on the amount of diene present.
U.S. Pat. No. 3,451,988 describes a polymerization catalyst for polymerizing vinyl compounds that utilizes mixtures of hydrocarbyl lithium and magnesium compounds, and a modifier which is a bi-functional Lewis base such as tertiary diamines including tetramethylethylene diamine.
U.S. Pat. No. 4,429,091 describes modifiers for the polymerization of dienes utilizing lithium-based initiators. The polymers and copolymers prepared in accordance with the procedure described in this patent contain from between 20 and 95% of the 1,2-microstructure. The catalyst system employed in the production of the polymers or copolymers is a combination of an anionic initiator based on lithium, and a modifier compound selected from the group consisting of linear and cyclic oligomeric oxolanyl alkanes.
U.S. Pat. No. 4,537,939 describes a catalyst system and process for the preparation of diene polymers and copolymers having a 1,2-microstructure content of between about 20 and about 65 percent. The catalyst system which is employed in the production of the polymers or copolymers is a combination of an anionic initiator based on lithium and a phosphine oxide modifier which is substituted with three heterocyclic rings, each hetero ring containing one nitrogen atom and either four, five or six carbon atoms.
U.S Pat. No. 4,591,624 describes a catalyst system and process for the preparation of diene polymers or copolymers having increased amounts of 1,2-microstructure. The catalyst system which is employed in the production of the polymers or copolymers is a combination of an anionic initiator based on lithium and a modifier composition comprising at least one cyclic acetal of a glyoxal.